Fix return type of w32_last_error.

src/w32fns.c (w32_last_error): Change the return value to DWORD, to
 match what GetLastError returns.  Explain better why the function is
 needed.

src/ChangeLog

@@ -1,5 +1,9 @@
 2012-10-09  Eli Zaretskii  <eliz@gnu.org>
 
+	* w32fns.c (w32_last_error): Change the return value to DWORD, to
+	match what GetLastError returns.  Explain why the function is
+	needed.
+
 	* frame.c (delete_frame): Rename local variable 'tooltip_frame' to
 	'is_tooltip_frame', to avoid confusion with its global namesake.
 

src/w32fns.c

@@ -6983,8 +6983,10 @@ w32_strerror (int error_no)
   return buf;
 }
 
-/* For convenience when debugging.  */
-int
+/* For convenience when debugging.  (You cannot call GetLastError
+   directly from GDB: it will crash, because it uses the __stdcall
+   calling convention, not the _cdecl convention assumed by GDB.)  */
+DWORD
 w32_last_error (void)
 {
   return GetLastError ();